Along with the developments of the optical fiber communications in recent years, there is an increasing demand for a glass optical fiber, and it is demanded to reduce the cost of the glass optical fiber. In this situation, it is required to increase the productivity of the glass optical fiber, and enhance the production efficiency. Therefore, it has been examined to increase the dimensions of the optical fiber glass preform and enhance the mass productivity.
A method for manufacturing the glass preform for optical fiber is well known in which a porous glass is produced by a VAD method (Vapor phase Axial Deposition method), and the porous glass is, for example, dehydrated, consolidated and vitrified. This VAD method involves injecting a glass material and a flame gas from a burner and depositing glass particles on a rotating quartz glass rod in an axial direction. And the porous glass formed by depositing glass particles is dehydrated, consolidated, vitrified, and drawn to produce a glass preform like the rod. Then, glass particles are further deposited on the outside of the glass preform like the rod to form a porous glass and then obtain a glass preform having a predetermined outer diameter.
In manufacturing the glass preform for optical fiber having a predetermined outer diameter, a quartz glass or a vitrified drawn glass is employed for the glass rod as the starting material for use in depositing the glass particles. The glass rod of the starting material has a different length depending on the dimensions of the glass preform to be produced, and if there is any abnormal part in the glass rod, it is required to remove the abnormal part.
Therefore, the long glass rod is parted in a predetermined length to obtain the glass rod having a length according to the glass preform to be produced, using the lathe machine.
FIG. 5 shows a conventional parting apparatus for parting the glass rod in a predetermined length. In the drawing, 1 is a glass rod, 2 is a lathe machine, 3 is a base board, 4 is a fixed supporter, 5 is a movable supporter, 6 is a chuck, and 7 is a glass burner. This apparatus is constituted such that one end portion of the glass rod 1 is grasped by the chuck 6 of the fixed supporter 4, and the other end portion is grasped by the chuck 6 of the movable supporter 5, as shown in FIG. 5.
And the movable supporter 5 is disposed so that its position can be adjusted on the base board 3 so as to correspond to a length of the glass rod 1. Also, the chuck 6 is rotatable with respect to the supporters 4, 5 and grasps the glass rod 1 rotatably.
The glass rod 1 with both ends grasped by the chucks 6 is rotated by rotation driving means (not shown), and heated and fused uniformly at a predetermined parting position by the glass burner 7.
Thereafter, the movable supporter 5 is moved to break a softened part that is heated and fused to part the glass rod 1 into two. Half pieces of the glass rod 1 parted into two are grasped in cantilever fashion by the chucks 6 respectively, but a stress concentrates on a grasped part of the glass rod 1. If the glass rod 1 is shorter, the stress is smaller, and the glass rod 1 is grasped in cantilever fashion without causing any problem.
However, when the glass rod 1 is longer, it is increased in weight, so that a larger stress in cantilever fashion causes a crack in the grasped part of the glass rod 1, possibly leading to a rupture.